Combustion of various chemical species involves a number of important considerations which can vary from one application to another. One recent area of interest in combustion chemistry is the enhancement of fuels for use in hypersonic jets and other high performance propulsion systems. In such applications, very fast fuel combustion is required for efficient propulsion. For example, in a ramjet or scramjet engine, the high volumetric air flow through the engine requires that combustion takes place on the millisecond or sub-millisecond time frame. This short combustion time frame is important so that combustion is substantially complete before the fuel-air mixture exits the engine. Unfortunately, much of the residence time is required for fuel-air mixing, making combustion on a substantially shorter time scale desirable. Another example of a high performance propulsion system is pulse-detonation engines (PDEs), where the fuel-air mixture combusts as a detonation wave propagates through the engine combustion chamber. Successful propagation of a detonation wave typically requires a sufficiently high rate of combustion to feed the combustion energy into driving the shock wave. The combustion time can depend on the engine tube diameter, but a typical value for the desired combustion time can be less than 100 microseconds. Further, current fuels have a limited energy density, especially when combined with materials having heats of combustion significantly below that of the fuel.
For high speed propulsion systems, the fuel and gas flow should be unimpeded. Thus, fixed catalyst beds are generally not suitable for use in such systems. Further, combustion rates must be increased, while ignition delays should be decreased. Any attempt to improve these aspects should also avoid substantial degradation of the energy density of the fuel.
In order to solve these and other problems, metallized propellants have become known in the art. These metallized propellants typically disperse aluminum or boron particles in a solid or liquid fuel, often as a gelled propellant. Aluminum and boron have high volumetric heats of combustion, relative to typical hydrocarbon fuels. As such, metallized propellants potentially have improved energy content relative to pure hydrocarbons. However, these metallized propellants can also present difficulties with respect to combustion times and efficiencies. These difficulties can reduce their usefulness for many combustion applications and particularly for applications requiring short combustion time scales. As such, materials and methods for improved high speed propulsion fuels and methods continue to be sought.